1. Field of the Invention
The present invention generally relates to automated optical inspection (AOI) systems and, more particularly, to automated inspection systems suitable for end-of-line semiconductor component manufacture and packaging, especially for inspection of metallic wirebond surfaces used with laminated chip carriers.
2. Description of the Prior Art
The art of semiconductor manufacturing has become sufficiently advanced that extremely complex devices can be fabricated at high integration density and very high manufacturing yield. Increases in chip integration density has required similar increases in chip carrier complexity and feature density to complement the advantages of reductions in connection length achieved on individual chips. In either case, the delicate metallurgical and chemical processes involved can be easily affected by contamination and other conditions which are not completely preventable and localized defects often occur. For this reason, sophisticated burn-in and testing arrangements and apparatus have been developed to assure full functionality and operational specifications of newly fabricated chips. Chip carriers, on the other hand, are subjected to inspection as well as functional testing prior to being populated with chips since many defect types which can occur in connections and pads cannot be detected by functional testing alone.
Inspection of chip carriers in various panel formats is commonly done prior to chip assembly to assure that reliable connections to the chips can be made by the package and support structure. In particular, for high reliability, wire bonding is currently preferred for making connections to chips although other techniques are known and in widespread use. Most of these techniques include the attachment of wires or leads to metallic connection pads and/or ring surfaces formed on the chip carrier. For a reliable connection to be made, connection pad or ring surfaces must be generally flat and free of contamination or voids (e.g. perforations in the pad) greater than a certain size. Variations from flatness (e.g. nodules and pits) and voids (e.g. holes extending through the metal layer) must be held within a closely controlled dimension to avoid compromise of the reliability of the connection or the reliability of the process by which the connection is made.
At the current state of the art, the wirebond surfaces are individually inspected by operators using low to medium power microscopes. Multiple inspections by different operators are considered to be required since inspection efficiency of individual operators is low. Further, the number of operators required for even modest production quantities engenders inconsistency in the inspection process. It can be readily understood that such an inspection process is labor-intensive and costly. The cost of such inspection thus adds significantly to the cost of the finished circuit packages while not insuring a maximal manufacturing yield.
Automated optical inspection (AOI) equipment that exists at the present time is intended for inspection of circuit traces on large panels. Imaging the traces and checking for the presence or absence of a trace or other feature is easily accomplished with these devices. With reflective white light inspection systems, some surface defects such as dishdowns (a substantial decrease in circuit trace height) may also be detected by such systems but these systems are not intended for more subtle surface imperfections and are inefficient and inaccurate when used for such a purpose. Further, the cameras used in these systems are line scan cameras providing only a single pass during scanning and thus are of limited flexibility relative to illumination of the object being inspected.
In this regard, the metallic surface of a wirebond pad is very difficult to illuminate in a consistent manner and may have variable surface textures due to minor process fluctuations which obscure certain types of defects to be detected. For example, acceptable surface roughness can create sufficient contrast under some lighting conditions that larger, unacceptable pits and nodules cannot be distinguished using simple segmentation thresholding methods.
Illumination cannot, as a practical matter, be altered consistent with the use of a line scan imaging device within an exposure in an automated inspection system. This prohibits inspection under multiple illumination conditions. Multiple imaging passes, in order to provide an alteration of illumination from exposure to exposure, is not considered a practical solution.
Further, AOIsystem generally image the entire surface of a part and transform either the test image or reference to align with the other for comparison through application of defect detection algorithms. The alignment process that must occur has drawbacks during the inspection process. First, it requires additional computation prior to the detection process which can become significant overhead for the system. Also, depending on the defect cetection methods used, slight pixel to pixel differences between test images and reference images of data that are otherwise unimportant can require detection sensitivity parameters to be lowered, thus lowering detectability for some types of defects.
Even with manual inspection by operators, identified potential defects must be verified, generally by another operator on another inspection machine. The transfer of a chip carrier having a suspected defect to another machine takes a significant amount of time, subjects the carrier to damage and inspection may be compromised by misalignment or misregistration in the further inspection machine. The change in operators is also a source of error.
In summary, inspection by operators is expensive and of low efficiency while existing automated optical inspection systems are ill-adapted to identify surface defects, particularly of metallic surfaces or other surfaces where texture may vary. Illumination cannot generally be varied and registration, alignment and detection errors may compromise the inspection process between machines and/or operators and even between exposures. Throughput of either manual or automated processes is extremely limited.